Electronic commutation of electric motor phase windings is used to control the torque produced and the resulting rotation of the motor shaft. Torque about the rotor shaft is produced from the interaction of magnetic force fields generated by a magnetic rotor attached to the motor shaft and current flowing through the stator phase windings. Maximum torque occurs when the angle between stator and rotor magnetic field vectors is 90 electrical degrees and decreases as the vectors align during rotation.
To control shaft rotation, the stator phase windings can be energized in a sequence defined by the angular position of the magnetic rotor with respect to the stator phase windings. Angular position of the magnetic rotor can be detected using three stationary Hall effect sensors positioned on a radius about the shaft and a multi-pole ring magnet aligned with the rotor poles attached to the shaft. The Hall effect sensor outputs can collectively be used to determine sequential commutation states during rotation of the magnetic rotor.
For example, in a stator having three phase windings, a six-step commutation method may produce six possible stator magnetic field vectors over 360 electrical degrees. Each commutation state may represent 60 electrical degrees with one phase winding connected to a positive voltage, a second phase winding connected to a negative voltage and a third phase winding not connected. A commutation event may be defined as occurring when the rotor moves from a position associated with one commutation state to a position associated with a next commutation state, as determined by a change in output from the Hall effect sensors due to rotation of the rotor ring magnet. For a new commutation state, the stator phase windings may be energized with the correct voltage polarities. In a PWM motor control system, phase winding voltage polarity and average value are typically controlled by comparison of a duty cycle value with a dual-sloped linear ramp function driven by a controller. The comparison output is valid for the time the dual-sloped linear ramp function value exceeds the duty cycle value.
Each phase winding can be driven by a complementary pair of electronic switching devices controlled by PWM drive signals generated by the controller. For each phase winding, three possible voltage connections can be made using the electronic switching device pair: positive voltage when the HI device is powered on, negative voltage when the LO device is powered on and no voltage when both devices are powered off. Simultaneously powering on the HI and LO devices of a phase winding may result in a short circuit, allowing potentially destructive shoot through currents to flow from the positive to negative voltage supplies through the electronic switching device pair. Shoot through currents can be avoided by incurring a dead-time between powering off one electronic switching device and powering on the other electronic switching device.
The timing diagram of FIG. 1 illustrates an ideal commutation response to a commutation event 10 represented by the transition from a first commutation state (State 1) to a second commutation state (State 2) for a three phase motor with a stator having three similar phase windings A, B and C. Prior to commutation event 10, the phase A terminal is connected to positive voltage, the phase B terminal is connected to negative voltage and the phase C terminal is unconnected. Upon duty cycle completion, the phase A terminal is momentarily disconnected to incur dead-time 12 to avoid shoot through current. The phase A terminal is then connected to negative voltage to allow freewheeling load current to circulate. A resultant voltage will be impressed across the phase A and phase B terminals in direct proportion to the PWM duty cycle.
Upon occurrence of commutation event 10, new voltage connections are required. The phase A terminal is disconnected, the phase B terminal remains connected to negative voltage and the phase C terminal is connected to positive voltage. A new PWM period or cycle is initiated and the average resultant voltage impressed across the phase C and phase B terminals would be equal to the average resultant voltage impressed across the phase A and phase B terminals for the previously completed PWM cycle prior to the commutation event. Although this ideal embodiment is technically feasible, many hardware implementations are incapable of implementing this ideal operation.
In some hardware implementations incapable of ideal operation, phase winding voltage polarities for a new commutation state cannot take effect until the current or ongoing PWM drive signal cycle is complete, thus delaying commutation. A six-step commutation method cannot maintain the angle between the rotor and stator magnetic fields at 90 electrical degrees for maximum torque. The actual angle varies from 60 to 120 electrical degrees. The commutation event is critical for its angular (time) accuracy and any deviation will cause torque ripple and speed variations.
The timing diagram of FIG. 2 illustrates typical delayed commutation from State 1 to State 2 due to hardware limitations. After commutation event 10, the voltage connections are changed only after initiation of the new PWM drive signal cycle at 14. While common, this method results in a delay of up to one PWM drive signal cycle.
The timing diagram of FIG. 3 illustrates a decreased resultant average voltage applied across the phase B and phase C terminals in response to commutation event 10. After commutation event 10, the ongoing PWM drive signal being applied to the phase A terminal is steered to the phase C terminal as shown. However, this technique results in application of an incorrect duty cycle to the new commutation state.
These delays (FIG. 2) and incorrect duty cycles (FIG. 3) in response to a change in commutation states create torque ripple and speed variations due to the deviation of the electrical angle between stator and rotor magnetic field vectors. There is a need for a motor system and method for its control that reduces or eliminates these shortcomings.